You must inform the housing benefit authority immediately if, not only temporarily

• the total income of all household members increases by more than 15 percent,
• your rent or your housing costs excluding heating costs are reduced by more than 15 percent or
• the number of household members is reduced because they
  • are no longer part of the household
  • are no longer taken into account for housing benefit.

If the changes are retroactive or become known retroactively, your entitlement to housing benefit will also be reduced retroactively.